This invention relates to an AC power generating system in which an AC power output is produced by conversion from an intermediate DC output.
WO98/28832 discloses a generating set which comprises an engine/generator which provides a variable voltage electrical output, this output being rectified and fed to a DC to DC converter, the output DC voltage of the DC to DC converter being monitored by a control circuit and serving as an intermediate DC output which is fed to an inverter to generate an AC output which supplies an external load. The DC to DC converter decouples or isolates the intermediate DC output from the fluctuations in the current and/or voltage output of the generator so that the apparatus is able to accommodate substantial variations in the output of the generator while maintaining the intermediate DC output within desired operating parameters. The DC to DC converter also serves to decouple or isolate the generator from variations in the load. An energy storage device, for example a capacitor, is connected in parallel with the DC link and thereby provides a short term energy reserve when the load applied to the DC link voltage varies suddenly. This energy storage device may be supplemented by other similar devices with respective control systems. In some cases such supplementary energy storage devices may be excluded. Instead a load anticipation circuit may be introduced to cope with high impact or step loads.
The concept of power generation by such a generating set which automatically varies its speed to match load variations at any time is to operate the prime mover at an optimum speed, based on efficiency or fuel economy or low noise or any other desirable parameter, while providing the required engine kW output to satisfy the load demand and any losses in the system. To achieve this, the control circuit includes various electronic controllers which monitor and control the intermediate DC output or link voltage across a capacitor, through a voltage sensor and a feedback signal voltage.
The measurement and control circuit of the generating set disclosed in WO98/28832 monitors the generator output voltage and current together with the DC link voltage and operates through a feedback control loop to vary the speed of the engine so as maintain the DC link voltage constant and limit the generator output current to a level determined a current reference signal.
U.S. Pat. No. 5,563,802 discloses an integrated power system which includes an engine/generator power supply integrated with storage batteries to provide household-like AC power. For a small load demand, the storage batteries are used to provide electricity. For a large load demand, the engine/generator starts to supply electricity. The engine/generator also recharges the storage batteries if the load demand is smaller than the engine/generator maximum load capacity. For a larger load demand, both the engine/generator and the storage batteries supply electricity in parallel. The engine speed is variable for various loads. A pulse-width modulated signal is generated which monitors the DC voltage of the high voltage DC bus. This arrangement provides the variable engine speed system by speeding up or slowing down as the system power requirements change so as to maintain the voltage on the high voltage DC bus substantially constant. The bigger the load, the higher the engine speed that is required to produce the necessary power and keep a constant DC voltage.
U.S. Pat. No. 5,198,698 discloses an auxiliary power supply system including an internal combustion engine driving an alternator the output of which is rectified and supplied to DC bus lines across which a storage battery is connected. The DC bus lines may supply a consuming device such as an uninterruptible power supply or telecommunications system. The voltage across the bus lines is sensed and when the voltage drops below a selected value, indicating that the consuming device is drawing power from the battery beyond a desired limit, the engine is turned on for a period of time to warm it up, during which the power from the generator is not supplied to the DC bus lines. Thereafter, the generator supplies power to the DC bus lines to supply the consuming device and partially recharge the battery until the consuming device is no longer drawing power, after which the engine is shut off. The power supply system includes a controller which monitors the intervals of time between turn-ons of the engine and if a selected period of time is exceeded, the engine is started without supplying power from the generator to the DC bus lines to allow the engine and generator to warm-up, enhancing maintenance of the system. The operating characteristics of the engine and generator are sensed and stored for access by an operator and warnings are provided if these conditions exceed acceptable limits.
One object of one aspect of this invention is to enable operation of such a generating set without there having to be an energy storage device connected in parallel with the DC link to provide a short term energy reserve when the load applied to the DC link voltage varies suddenly.
According to one aspect of this invention there is provided an AC power generating system including inverter means which are operable to convert an intermediate DC voltage into an AC power output, means for generating and maintaining the intermediate DC voltage at one level and voltage sensor means operable to monitor the intermediate DC voltage, wherein control means are provided for the inverter means, said control means being responsive to the voltage sensor means and being operable to control operation of the inverter means so the voltage and/or the frequency of the AC power output is/are reduced in response to a fall in the intermediate DC voltage to a certain level caused by the application of a step load to the AC power output, whereby to provide a transient off-loading effect which will give the generating means time to respond and thereby allow the intermediate DC voltage to be restored to said one level.
The means for generating and maintaining the intermediate DC voltage of a preferred embodiment of this aspect of the invention include generator means operable to generate a variable voltage AC power supply and rectifier means having an output and operable to rectify the variable voltage AC power supply to establish the intermediate DC voltage.
Conveniently the generator means are driven by a variable speed prime mover, the control means of the generator means comprising speed control means operable to control the speed of the prime mover. Preferably the generator means-are a prime mover driven permanent magnet generator.
If the neutral of the AC output of the generating set disclosed by WO98/28832 were to be connected to the neutral terminal of the generator, the variable voltage three-phase output of the generator would have to be rectified by a half wave rectifier. As a result large and costly inductors would need to be used for the intermediate DC link. Furthermore, there would be a risk that a DC component could be established in the windings of the generator. This could result in torque pulsations due to the DC component of the generator current.
U.S. Pat. No. 4,507,724 discloses a polyphase inverter system which converts DC power developed by a DC source into polyphase AC power to drive a load. The DC source may be a generator and rectifier arrangement which includes control and protection circuitry for operating the generator. In any event, the DC source includes two terminals at first and second voltages and a neutral or common terminal which is at a voltage midway between the first and second voltages. The neutral of the polyphase AC power developed by the inverter is connected to the neutral or common terminal of the DC source.
U.S. Pat. No. 3,775,663 discloses a solid state inverter having an electronic neutral which is an additional single phase inverter operated as a current source or sink to maintain the potential at the neutral terminal at a controlled voltage independent of neutral current flow into and out of the neutral terminal. The inverter has input terminals which are adapted to be connected to a battery, a rectifier energized by an alternating voltage or some other source of unidirectional voltage.
A neutral setting of the AC power output could be generated and regulated by a controlled division of the intermediate DC voltage in which case the rectifier means could be full wave rectifier means. Accordingly a full-wave ripple voltage could be maintained in the DC link so that establishment of a DC component in the windings of the permanent magnet generator could be avoided.
Booster means having an input which is connected to the output of the rectifier means and which are operable to increase the voltage of the rectified output of said rectifier means and thereby to establish the intermediate DC voltage may be provided. The generator means may include respective control means and the voltage sensor means may provide a feedback control signal to the control means of the generator means whereby to effect variation of the variable AC power supply and thereby to counter a tendency of the intermediate DC voltage to vary.
In a preferred embodiment, current sensor means operable to monitor a DC load current caused by connection of a load across the intermediate DC voltage are provided, the current sensor means being operable to emit a signal which is indicative of the monitored DC load current, comparator means being provided for comparing an output signal from the current sensor means with a reference signal and for emitting a speed correction signal which is proportional to the amount by which the signal from the current sensor means exceeds the reference, that output from the comparator means being supplied to the speed control means of the generator means to effect an increase in the speed of the prime mover above that required for the applied load.
The AC power generating system may include brake control means and means responsive to the output of the voltage sensor means that is operable to monitor the intermediate DC voltage whereby to connect the brake control means across the intermediate DC voltage to apply a load thereto when the intermediate DC voltage rises to a certain level. The control means for the inverter means may be operable to increase the frequency of the AC power output in response to an increase in the intermediate DC voltage to a certain high level. The certain high level conveniently is higher than the level at which the brake control means is connected across the intermediate DC voltage.
In order to provide a short term supply of electrical energy when the DC link voltage tends to fall in response to a high load demand electrical energy storage means may be connected across the intermediate DC voltage so as to be charged by the intermediate DC voltage, and bistable switching means and associated control means may be provided, said bistable switching means normally being in one state in which they are operable to make the connection of the electrical energy storage means across the intermediate DC voltage to enable the electrical energy storage means to be charged and to interrupt a connection between the electrical energy storage means and the connection between the output of the rectifier means and the input of the booster means, said bistable switching means being operable in its other state to isolate the electrical energy storage means from the intermediate DC voltage and to make said connection of the electrical energy storage means with the connection between the output of the rectifier means and the input of the booster means whereby to enable discharge of electrical energy from said electrical energy storage means into the connection between the rectifier means and the booster means, said control means associated with the bistable switching means being responsive to the monitored intermediate DC voltage and being operable in response to a fall in said intermediate DC voltage to a certain reference level to switch said bistable switching means from its normal said one state to said other state whereby to discharge electrical energy to augment the rectified output of said rectifier means and thereby to counter the sensed fall in said intermediate DC voltage.
Hence the current to be supplied by the electrical energy storage means to augment the rectified output of the rectifier means need not be as large as it would need to be were it to be supplied to the DC link between the booster means and the inverter means as in the system disclosed in WO98/28832.
Varying the speed of the engine with changes in the load by using a feedback signal derived from monitoring the intermediate DC link voltage as taught by WO98/28832 or by U.S. Pat. No. 5,563,802 or by monitoring either the voltage or current of the variable voltage electrical output of the engine driven generator as taught by WO98/28832 is liable to result in unstable operation such as hunting because the sensed parameter does not respond directly to the external load.
WO98/07224 discloses a power conversion system which responds to an abnormal output condition such as overcurrent by one or more of a number of techniques including reducing engine speed. The system includes a control circuit, an engine, a generator, a controlled rectifier that provides a rectified signal, a power converter and a detector. It is a fault protection system
GB-A-2.318,000 discloses a UPS system with a battery and an inverter. It includes a microprocessor controlled monitoring facility which measures the voltages and currents including current fed to a power converter so that necessary adjustments and/or warnings can be made. The output of the inverter is regulated and stabilized.
An object of another aspect of this invention is to provide means for varying the speed of the engine with changes in the external load which operate in a stable manner.
According to that other aspect of the invention, there is provided an AC power generating system including generator means operable to generate a variable voltage AC power supply, rectifier means having an output and being operable to rectify the variable voltage AC power supply, booster means having an input which is connected to the output of the rectifier means and which are operable to increase the voltage of the rectified output of said rectifier means to establish an intermediate DC voltage, a variable speed prime mover drivingly coupled with the generator means, and speed control means operable to control the speed of the prime mover, wherein current sensor means are provided, said current sensor means being operable to monitor a DC load current caused by connection of a load across the intermediate DC voltage, the current sensor means being operable to emit a signal which is indicative of the monitored DC load current, comparator means being provided for comparing an output signal from the current sensor means with a reference signal and for emitting a speed correction signal which is proportional to the amount by which the output signal from the current sensor means exceeds the reference signal, the output from the comparator means being supplied to the speed control means for the generator means to effect an increase in the speed of the variable speed prime mover.
An object of a further aspect of this invention is to provide an improved arrangement for providing a short term supply of electrical energy when the DC link voltage tends to fall in response to a high load demand.
According to the further aspect of this invention there is provided, an AC power generating system including generator means operable to generate a variable voltage AC power supply, rectifier means having an output and operable to rectify the variable voltage AC power supply, booster means having an input which is connected to the output of the rectifier means and which is operable to increase the voltage of the rectified output of said rectifier means and thereby to provide an intermediate DC voltage, inverter means operable to convert the intermediate DC voltage into an AC power output for supplying to an external load, voltage sensor and control means operable to monitor the intermediate DC voltage and to provide a feedback control signal to the generator means whereby to vary the variable voltage AC power supply so as to counter a tendency of the intermediate DC voltage to vary, and electrical energy storage means connected across the intermediate DC voltage so as to be charged by the intermediate DC voltage, wherein bistable switching means and associated control means are provided, said bistable switching means normally being in one state in which they are operable to make the connection of the electrical energy storage means across the intermediate DC voltage to enable the electrical energy storage means to be charged and to interrupt a connection between the electrical energy storage means and a connection between the generator means and the input of the booster means, said bistable switching means being operable in its other state to isolate the electrical energy storage means from the intermediate DC voltage and to make said connection of the electrical energy storage means with the connection between the generator means and the input of the booster means, whereby to enable discharge of electrical energy from said electrical energy storage means into the connection between the generator means and the booster means, said control means associated with the bistable switching means being responsive to the monitored intermediate DC voltage and being operable in response to a fall in said intermediate DC voltage to a certain reference level to switch said bistable means from its normal said one state to said other state whereby to discharge electrical energy to augment the variable voltage AC power supply and thereby to counter the sensed fall in said intermediate DC voltage.
Preferably the connection between the generator means and the input of the booster means is the connection between the rectifier means and the input of the booster means so that the discharged electrical energy augments the rectified output of said rectifier.
A neutral setting of the AC power output could be generated and regulated by a controlled division of the intermediate DC link voltage before it is converted into the AC power output in which case the connection between the electrical energy storage means and the connection between the output of the rectifier means and the input of the booster means need only be made with one of the positive and negative connections between the rectifier means and the booster means so that the number of components required is reduced and cost is saved.
An object of another aspect of this invention is to enable smaller, less expensive inductors to be used in the DC link.
Although generation and regulation of a neutral setting of the AC power output by a controlled division of the voltage of the DC power before that power is converted to AC would enable the use of smaller less expensive inductors, it has the drawback that the capacitors that are connected in series across the DC voltage to effect that division are also connected across the load and therefore must be highly rated so that each of them is capable of withstanding the full load to avoid the danger of capacitor damage in the event of one of them being short circuited. Furthermore, although the power generating system is designed so that the intermediate DC voltage is decoupled or isolated from fluctuations in the current and/or voltage output of the generator so that the apparatus is able to accommodate substantial variations in the output of the generator whilst maintaining the intermediate DC output within desired operating parameters, there are liable to be transient variations in the latter with variations in the load. Hence there may be transient variations in a neutral setting produced by a controlled division of the intermediate DC voltage. As stated above, U.S. Pat. No. 4,507,724 discloses a DC source which develops DC power which is converted into polyphase AC power, the DC source including two terminals at first and second voltages and a neutral or common terminal which is at a voltage midway between the first and second voltages.
Accordingly, an object of another aspect of this invention is to enable the use of full wave rectification whilst providing a reliably constant neutral setting which will serve as a good reference potential and which will reduce the need for expensive, highly-rated capacitors.
According to yet another aspect of this invention there is provided an AC power generating system including variable voltage AC power supply generator means, rectifier means having an output and operable to rectify the variable voltage AC power supply to establish an intermediate DC voltage, inverter means which are operable to convert the intermediate DC voltage into an AC power output, voltage sensor means operable to monitor the intermediate DC voltage and control means responsive to an output from the voltage sensor means and operable to maintain the intermediate DC voltage at one level, wherein the generator means is operable to generate two variable voltage AC power supplies, the rectifier means comprise two full-wave rectifiers, each full-wave rectifier being operable to rectify a respective one of the two variable voltage AC power supplies and each being connected on one side to a neutral terminal and having an output voltage terminal on its other side, the output voltage terminal of one of the full-wave rectifiers being positive and the output voltage terminal of the other full-wave rectifier being negative such that the two variable voltage AC power supplies are severally rectified to produce a positive and a negative potential respectively which together comprise the intermediate DC voltage, the voltage sensor means comprising two voltage sensors severally responsive to a respective one of the positive and negative output potentials of the two full-wave rectifiers and the control means comprising two controllers severally operable to maintain each of those positive and negative potentials at a certain level whereby to maintain the intermediate DC voltage at said one level.
Preferably there are two booster circuits, each connected between a respective one of the output voltage terminals of the two full-wave rectifiers and the neutral terminal and each operable to increase the voltage of the respective output potential and thus to increase the intermediate DC voltage, each of the controllers being operatively associated with the respective one of the booster circuits.
The generator means which may be a permanent magnet generator, may be driven by a variable speed prime mover which may be provided with speed control means operable to control the speed of the prime mover. Preferably current sensor means operable to monitor a DC load current caused by connection of a load across the intermediate DC voltage are provided, the current sensor means being operable to emit a signal which is indicative of the monitored DC load current, comparator means being provided for comparing an output signal from the current sensor means with a reference signal and for emitting a speed correction signal which is proportional to the amount by which the signal from the current sensor means exceeds the reference, that speed correction signal being supplied to the speed control means for the generator as a feed back control signal whereby to effect variation of the variable AC power supply and thereby to counter any tendency of the intermediate voltage to vary.
The AC power generating system may include brake control means and means responsive to the output of the two voltage sensors that are operable to monitor the positive and negative output potentials whereby to connect the brake control means across the intermediate DC voltage to apply a load thereto when the potential difference between the monitored positive and negative potentials rises to a certain level. Preferably one pair of capacitors are connected in parallel between the positive output potential and the neutral terminal, another pair of capacitors is connected in parallel between the negative output potential and the neutral terminal and the brake control means are connected across the positive and negative output potentials between the capacitors of each pair. Preferably the current sensor means that are operable to monitor the DC load current caused by connection of a load across the intermediate DC voltage are connected in either the positive or the negative connection between the brake control means and the capacitors of each pair that are remote from the two full-wave rectifiers and that are nearer to the inverter means.
The power supply system disclosed in U.S. Pat. No. 5,563,802 discussed above has microprocessor control means for integrating the battery and the engine/generator according to a predetermined control logic. The microprocessor monitors various parameters. It shuts down the engine/generator if certain parameters are exceeded. For example:
1) the speed of the engine/generator exceeds a predetermined limit (overspeed controlling);
2) the output of the inverter circuitry exceeds a predetermined limit (called chopper overvoltage);
3) the voltage output of the engine/generator exceeds a predetermined limit (called engine generator overvoltage); and
4) the boost voltage at the input of the inverter circuitry exceeds a predetermined limit (called booster overvoltage).
The microprocessor also activates the battery voltage booster upon a current sensor detecting a transient condition caused by addition of electrical load so as to overcome undershoots and activates charging of the battery upon the current sensor detecting a transient condition caused by deletion of an electrical load so as to overcome overshoots.
U.S. Pat. No. 5,606,244 discloses an AC power generating system which includes an engine driven generator, a rectifier, a booster which boosts the rectified output of the engine/generator if it is less than a predetermined value so as to generate a stable DC voltage at a suitable level which it feeds it to an inverter to produce an AC output. The voltage fed to the inverter is monitored and used as a feedback signal for the booster. The system includes electronic protection against over current, over load, short circuit and thermal over load conditions.
As stated above, U.S. Pat. No. 5,198,698 discloses an auxiliary power supply which supplies DC voltage to a consuming device such as a UPS system. G-A-2,318,000 and U.S. Pat. No. 5,811,960 disclose UPS systems, the latter being a battery-less system.
According to a further aspect of this invention, there is provided a method of operating an AC power generating system including the steps of:
(i) operating a variable speed prime mover driven generator to generate a variable voltage AC power supply;
(ii) rectifying the, variable voltage AC power supply to establish an intermediate DC voltage;
(iii) controlling the intermediate DC voltage to maintain it at a substantially constant level;
(iv) operating an inverter to convert the intermediate DC voltage into an AC power-output;
(v) monitoring the intermediate DC voltage;
(vi) comparing the intermediate DC voltage with a first reference voltage which is lower than the substantially constant level; and
(vii) controlling operation of the prime mover driven generator by feedback control so as to increase its speed when the monitored intermediate DC voltage falls to the level of the first reference voltage whereby to restore the intermediate DC voltage to said substantially constant level; wherein
(viii) the intermediate DC voltage is also compared with a second reference voltage which is lower than said first reference voltage, and
(ix) operation of the inverter is controlled so as to reduce the voltage and/or frequency of the AC power output when the monitored intermediate DC voltage falls to the level of the second reference voltage whereby to provide a transient off-loading effect which assists restoration of the intermediate DC voltage to said substantially constant level.
Preferably the method of operating an AC power generating system includes the further steps of:
a) monitoring a DC load current which results from connection of a load across the intermediate DC voltage;
b) comparing the monitored DC load current with a certain current reference level; and
c) further controlling operation of the prime mover driven generator by feedback control when the monitored DC load current exceeds the certain current reference level whereby to further increase the speed of the prime mover driven generator by an amount which is proportional to the amount by which the monitored DC load current exceeds the certain current level.
Operation of the inverter may further be controlled so as to increase the frequency of the AC power supply when the monitored DC voltage rises to the level of a third reference voltage. A brake controller may be connected across the monitored DC voltage to apply a load thereto when the monitored DC voltage rises to a certain voltage level. Conveniently the third reference voltage is higher than said certain voltage level.
In order to convert a DC voltage into a multiphase AC power output, it is usual to provide a pair of transistors for each phase, that pair of transistors being arranged so that the collector of one of them is connected to a positive of the DC voltage, the emitter of the other is connected to a negative of the DC voltage and the emitter of said one transistor is connected to the collector of said other transistor. The connection between the two transistors for each phase is connected through an inductance, or choke of an LC output filter to an AC power output terminal for the respective phase. Operation of the transistors is controlled by a controller which generates a pulse width modulated drive to the base of the respective transistor to control its operation. The basis of this control involves operating the transistors so as to connect the positive and negative potentials of the DC voltage alternately to the respective AC power output terminal through the inductance or choke of the LC output filter that smoothes it. This converter arrangement is adapted so as to minimise the number of power electronic devices and microprocessor controllers that are used. However the high frequency switching of the voltage between positive and negative potentials results in the production of a high ripple current. This ripple current is passed through the transistors, inductances or chokes, the LC output filter capacitors and the capacitors that are connected across the DC voltage. As a result it is necessary to employ high capacity DC capacitors in order to cater for the high ripple current. Furthermore ripple currents which oscillate between positive and negative current values result in bi-directional power flow between the capacitors that are connected across the DC voltage and the capacitors of the LC output filters.
An object of another aspect of this invention is to provide an arrangement for converting a DC voltage to an AC power output in which lower ripple currents are generated.
According to yet another aspect of this invention there is provided a method of converting a DC voltage having a positive and negative level of electrical potential into an AC power output in which the positive and negative levels of electrical potential are connected to an AC power output terminal alternately, wherein each connection of one of the positive and negative levels of electrical potential to the output terminal is separated in time from a connection of the other of the positive and negative levels of the electrical potential to the output terminal by an intermediate period of zero voltage.
Where the source of each of the positive and negative potentials of DC voltage is a respective charged capacitor and the positive and negative levels of electrical potential are connected alternately to the AC power output terminal through an output filter, both sides of the output filter may be connected to neutral for said intervening periods.
According to a further aspect of this invention there is provided a DC to AC converter including a first source of electrical energy operable to be charged with a positive DC potential, a second source of electrical energy operable to be charged with a negative DC potential, first bistable switching means operable in one state to connect the first electrical energy source to an AC power output terminal through an inductor means of an output filter means and to break that connection in its other state, second bistable switching means operable in one state to connect the second electrical energy source to the AC power output terminal through said inductor means and to break that connection in its other state, third bistable switching means operable in one state to connect said inductor means to neutral and to break that connection in its other state, and control means operable to control operation of said first, said second and said third bistable switching means so that said first and said second bistable switching means are switched to their said one state alternately and are in their said other state when the other of said first and second bistable switching means are in their said one state and said third bistable switching means is switched to its said one state whilst said first and second bistable switching means are both switched to their said other state between each alternate switching of said first and second bistable switching means to their said one state so that the positive and negative levels of electrical potential with which said first and second electrical energy sources are charged when said converter is operated are connected to the AC power output terminal alternately and each connection of the one of the positive and negative levels of electrical potential to the output terminal is separated in time from a connection of the other of the positive and negative levels of electrical potential to the output terminal by an intervening period of zero voltage.
Preferably the output filter means includes a third chargeable source of electrical energy which is connected on one side to the inductor means and on its other side to neutral, the third bistable switching means being operable to connect the other side of the inductor means to neutral when in its said one state.
In a preferred embodiment of this invention the DC to AC converter is the inverter means of an AC power generating system which includes variable voltage AC power supply generating means, rectifier means having an output and operable to rectify the variable voltage AC power supply to establish an intermediate DC voltage, the inverter means being operable to convert the intermediate DC voltage into the AC power output, there being voltage sensor means operable to monitor the intermediate DC voltage and control means responsive to an output of the voltage sensor means and operable to maintain the intermediate DC voltage at one level. Preferably the generator means is operable to generate two variable voltage AC power supplies and two full wave rectifiers are provided, each being operable to rectify a respective one of the two variable voltage AC power supplies and each being connected on one side to a neutral terminal and having an output terminal on its other side, the output terminal of one of the fullwave rectifiers being positive and the output terminal of the other fullwave rectifier being negative such that the two variable voltage AC power supplies are rectified to produce a positive and a negative potential respectively which together comprise the intermediate DC voltage and which are respectively connected across the first and second electrical energy storage means, there being voltage sensor means severally responsive to a respective one of the positive and negative output potentials of the two fullwave rectifiers and control means operable to maintain each of those positive and negative potentials at a certain level whereby to maintain the intermediate DC voltage at one level.
Preferably there are two booster circuits, each connected between a respective one of the output terminals of the two fullwave rectifiers and the neutral terminal and each operable to boost the respective one of the output electrical potentials, each of the control means being operatively associated with a respective one of the booster circuits.
According to yet another aspect of this invention there is provided a method of operating an AC power generating system of the kind which includes an engine driven generator which provides a variable voltage electrical output, rectifier means operable to rectify the variable voltage electrical output, booster means operable to boost the voltage of the variable voltage electrical output and thereby to provide an intermediate DC voltage, inverter means which are operable to convert the intermediate DC voltage into an AC power output, means operable to maintain the intermediate DC voltage at one level up to a predetermined speed of the engine driven generator which is less than the maximum speed thereof, wherein the means operable to maintain the intermediate DC voltage at said one level is disenabled when the speed of the engine driven generator rises to at least said predetermined speed and the speed of the engine driven generator and the intermediate DC voltage are allowed to rise so that, when the engine driven generator is running at maximum speed, the AC power output is allowed to increase until a balance is achieved.